The DataInput interface provides
for reading bytes from a binary stream and
reconstructing from them data in any of
the Java primitive types. There is also
a
facility for reconstructing a String
from data in
modified UTF-8
format.

It is generally true of all the reading
routines in this interface that if end of
file is reached before the desired number
of bytes has been read, an EOFException
(which is a kind of IOException)
is thrown. If any byte cannot be read for
any reason other than end of file, an IOException
other than EOFException is
thrown. In particular, an IOException
may be thrown if the input stream has been
closed.

Implementations of the DataInput and DataOutput interfaces represent
Unicode strings in a format that is a slight modification of UTF-8.
(For information regarding the standard UTF-8 format, see section
3.9 Unicode Encoding Forms of The Unicode Standard, Version
4.0).
Note that in the following table, the most significant bit appears in the
far left-hand column.

All characters in the range '\u0001' to
'\u007F' are represented by a single byte:

Bit Values

Byte 1

0

bits 6-0

The null character '\u0000' and characters
in the range '\u0080' to '\u07FF' are
represented by a pair of bytes:

Bit Values

Byte 1

1

1

0

bits 10-6

Byte 2

1

0

bits 5-0

char values in the range '\u0800'
to '\uFFFF' are represented by three bytes:

Bit Values

Byte 1

1

1

1

0

bits 15-12

Byte 2

1

0

bits 11-6

Byte 3

1

0

bits 5-0

The differences between this format and the
standard UTF-8 format are the following:

The null byte '\u0000' is encoded in 2-byte format
rather than 1-byte, so that the encoded strings never have
embedded nulls.

Method Detail

readFully

Reads some bytes from an input
stream and stores them into the buffer
array b. The number of bytes
read is equal
to the length of b.

This method blocks until one of the
following conditions occurs:

b.length
bytes of input data are available, in which
case a normal return is made.

End of
file is detected, in which case an EOFException
is thrown.

An I/O error occurs, in
which case an IOException other
than EOFException is thrown.

If b is null,
a NullPointerException is thrown.
If b.length is zero, then
no bytes are read. Otherwise, the first
byte read is stored into element b[0],
the next one into b[1], and
so on.
If an exception is thrown from
this method, then it may be that some but
not all bytes of b have been
updated with data from the input stream.

Parameters:

b - the buffer into which the data is read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

readFully

len bytes
of input data are available, in which case
a normal return is made.

End of file
is detected, in which case an EOFException
is thrown.

An I/O error occurs, in
which case an IOException other
than EOFException is thrown.

If b is null,
a NullPointerException is thrown.
If off is negative, or len
is negative, or off+len is
greater than the length of the array b,
then an IndexOutOfBoundsException
is thrown.
If len is zero,
then no bytes are read. Otherwise, the first
byte read is stored into element b[off],
the next one into b[off+1],
and so on. The number of bytes read is,
at most, equal to len.

Parameters:

b - the buffer into which the data is read.

off - an int specifying the offset into the data.

len - an int specifying the number of bytes to read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

skipBytes

Makes an attempt to skip over
n bytes
of data from the input
stream, discarding the skipped bytes. However,
it may skip
over some smaller number of
bytes, possibly zero. This may result from
any of a
number of conditions; reaching
end of file before n bytes
have been skipped is
only one possibility.
This method never throws an EOFException.
The actual
number of bytes skipped is returned.

readByte

Reads and returns one input byte.
The byte is treated as a signed value in
the range -128 through 127,
inclusive.
This method is suitable for
reading the byte written by the writeByte
method of interface DataOutput.

Returns:

the 8-bit value read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

readUnsignedByte

Reads one input byte, zero-extends
it to type int, and returns
the result, which is therefore in the range
0
through 255.
This method is suitable for reading
the byte written by the writeByte
method of interface DataOutput
if the argument to writeByte
was intended to be a value in the range
0 through 255.

Returns:

the unsigned 8-bit value read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

readFloat

Reads four input bytes and returns
a float value. It does this
by first constructing an int
value in exactly the manner
of the readInt
method, then converting this int
value to a float in
exactly the manner of the method Float.intBitsToFloat.
This method is suitable for reading
bytes written by the writeFloat
method of interface DataOutput.

Returns:

the float value read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

readDouble

Reads eight input bytes and returns
a double value. It does this
by first constructing a long
value in exactly the manner
of the readLong
method, then converting this long
value to a double in exactly
the manner of the method Double.longBitsToDouble.
This method is suitable for reading
bytes written by the writeDouble
method of interface DataOutput.

Returns:

the double value read.

Throws:

EOFException - if this stream reaches the end before reading
all the bytes.

readLine

Reads the next line of text from the input stream.
It reads successive bytes, converting
each byte separately into a character,
until it encounters a line terminator or
end of
file; the characters read are then
returned as a String. Note
that because this
method processes bytes,
it does not support input of the full Unicode
character set.

If end of file is encountered
before even one byte can be read, then null
is returned. Otherwise, each byte that is
read is converted to type char
by zero-extension. If the character '\n'
is encountered, it is discarded and reading
ceases. If the character '\r'
is encountered, it is discarded and, if
the following byte converts to the
character '\n', then that is
discarded also; reading then ceases. If
end of file is encountered before either
of the characters '\n' and
'\r' is encountered, reading
ceases. Once reading has ceased, a String
is returned that contains all the characters
read and not discarded, taken in order.
Note that every character in this string
will have a value less than \u0100,
that is, (char)256.

Returns:

the next line of text from the input stream,
or null if the end of file is
encountered before a byte can be read.

readUTF

Reads in a string that has been encoded using a
modified UTF-8
format.
The general contract of readUTF
is that it reads a representation of a Unicode
character string encoded in modified
UTF-8 format; this string of characters
is then returned as a String.

First, two bytes are read and used to
construct an unsigned 16-bit integer in
exactly the manner of the readUnsignedShort
method . This integer value is called the
UTF length and specifies the number
of additional bytes to be read. These bytes
are then converted to characters by considering
them in groups. The length of each group
is computed from the value of the first
byte of the group. The byte following a
group, if any, is the first byte of the
next group.

If the first byte of a group
matches the bit pattern 0xxxxxxx
(where x means "may be 0
or 1"), then the group consists
of just that byte. The byte is zero-extended
to form a character.

If the first byte
of a group matches the bit pattern 110xxxxx,
then the group consists of that byte a
and a second byte b. If there
is no byte b (because byte
a was the last of the bytes
to be read), or if byte b does
not match the bit pattern 10xxxxxx,
then a UTFDataFormatException
is thrown. Otherwise, the group is converted
to the character:

(char)(((a & 0x1F) << 6) | (b & 0x3F))

If the first byte of a group
matches the bit pattern 1110xxxx,
then the group consists of that byte a
and two more bytes b and c.
If there is no byte c (because
byte a was one of the last
two of the bytes to be read), or either
byte b or byte c
does not match the bit pattern 10xxxxxx,
then a UTFDataFormatException
is thrown. Otherwise, the group is converted
to the character:

(char)(((a & 0x0F) << 12) | ((b & 0x3F) << 6) | (c & 0x3F))

If the first byte of a group matches the
pattern 1111xxxx or the pattern
10xxxxxx, then a UTFDataFormatException
is thrown.

If end of file is encountered
at any time during this entire process,
then an EOFException is thrown.

After every group has been converted to
a character by this process, the characters
are gathered, in the same order in which
their corresponding groups were read from
the input stream, to form a String,
which is returned.

The writeUTF
method of interface DataOutput
may be used to write data that is suitable
for reading by this method.

Returns:

a Unicode string.

Throws:

EOFException - if this stream reaches the end
before reading all the bytes.